An organic molecule containing organic ring structures and a chain structure threading through the organic ring structures allows the organic ring structures and the chain structure respectively to have their own functions. As an example of such an organic molecule, a polyrotaxane having two or more rotaxane structures in a single molecule is known. Polyrotaxanes are expected to have a wide variety of applications in the medical, chemical, and electronics fields.
As a method of synthesizing polyrotaxanes, a method of utilizing the hydrophobicity of the interior of the ring structure of a cyclodextrin and the hydrophilicity of the exterior thereof is known. In one example of this method, cyclodextrins and chain-like organic molecules having poor water solubility are mixed in an aqueous solvent. A method is known in which both ends of a guest molecule are modified or substituted with bulky molecules so that the guest molecule is end-capped in order to prevent dethreading of cyclodextrins (A. Harada, J. Li, & M. Kamachi, “Nature”, 356, 325 (1992)). It should be noted that a polyrotaxane whose both ends are not capped is sometimes called a pseudo-polyrotaxane.
In the above-mentioned synthesis method, it is difficult to control the amount of cyclodextrins to be used for forming rotaxane structures. Therefore, this method has a problem that the solubility of the resulting polymer decreases due to hydrogen bonding of hydroxyl groups on adjacent cyclodextrins as the amount of cyclodextrins increases, which hinders the progress of the reaction in the solvent. As a countermeasure against this problem, a method is proposed in which hydrophobic monomers included in cyclodextrins and hydrophilic monomers are polymerized alternately with each other by a Suzuki coupling reaction (Harry L. Anderson, et al., Angewandte Chemie International Edition, 39, 3456-3460 (2000)).
The above-mentioned synthesis methods both are reactions carried out in aqueous solvents using cyclodextrins. On the other hand, a method of synthesizing pseudo-polyrotaxanes in an organic solvent by using permethylcyclodextrins obtained by substituting hydroxyl groups of cyclodextrins with methoxy groups is proposed (M. Okada, M. Kamachi, & A. Harada, Macromolecules, 32, 7202 (1999)). A method of synthesizing polyrotaxanes by a solid-state reaction by mixing an end-capping agent with pseudo-polyrotaxanes under pressure (JP 2005-75979 A).
In the above-mentioned synthesis methods of polyrotaxanes described in the documents written by Harada, et al. and Okada, et al., and JP 2005-75979 A, however, a previously synthesized chain-like polymer is included in cyclodextrins or permethylcyclodextrins, which causes a problem that the amount of cyclodextrins or permethylcyclodextrins to be used for the inclusion cannot be controlled. Furthermore, molecules that can be included as guest molecules by the method of Okada, et al. are limited to those of polypropylene glycol, polytetrahydrofuran, etc., as disclosed in Okada, et al. Therefore, this method has a problem that conjugated polymers or the like, which can be used as conductive polymers, cannot be used as guest molecules.
The methods of Harada, et al. and Anderson, et al. are carried out in aqueous solvents by using cyclodextrins. In these methods, water molecules are attracted to hydroxyl groups of cyclodextrins that are hydrophilic functional groups and ionic functional groups of the principal chain thereof, which results in the incorporation of water into a reaction product at the molecular level. Since it is difficult to remove these water molecules, polyrotaxanes synthesized by these methods can hardly be used in applications such as electronics, which are susceptible to adverse effects of water and ions. As a solution of this problem, it is conceivable to substitute the hydrophilic functional groups of the synthesized polyrotaxanes with hydrophobic functional groups. This method is, however, impractical because of its poor reaction efficiency in substituting all the hydrophilic functional groups with hydrophobic functional groups in a polymer state.